High-availability data storage systems require multiple power supplies which are used to supply redundant power to certain components of the data storage system via, in particular the disk drive array, via a common power bus. Each power supply may also supply power to other subsystem components in a non-redundant manner. In order to prevent faults in one of the power supplies or subsystem components from adversely affecting the performance of the components in the rest of the data storage system, these systems typically include fault protection devices which prevent faults in the common load from affecting the operation of the power supplies and which prevent faults in a power supply from affecting the operation of the other subsystem components.
FIG. 1 is a schematic diagram of a prior art power supply system 100 including a plurality of power supplies 102A, 102B, 102N for supplying redundant power to a common load 104 via common power bus V100. Each of power supplies 102A, 102B, 102N also supply power in a non-redundant manner to loads 106A, 106B and 106N, respectively. Each branch 108A-108N, which delivers power from each power supply 102A-102N to the common power bus V100 includes fault protection devices 110A-110N and 116A-116N, respectively, which are connected to their respective power supplies 102A-102N via power supply lines V100A-V100N.
Fault protection devices 110A-110N provide over-current protection which prevents a fault in the common load from adversely affecting the operation of the associated power supply and the subsystem component loads 106 connected to the associated power supply. For example, a short in the common load would cause an excessive forward current in the power supply lines V100A-V100N. This excessive forward current in supply lines V100A-V100N could cause the associated power lines (V200A-V200N and/or V300A-V300N and/or Vn_A-Vn_N) to either droop or cease to operate altogether if the power supply has internal overcurrent protection. Over-current protection devices 110A-110N may include a passive device, such as a fuse 112A-112N, which trips when the current in power supply line V100A-V100N exceeds a reference value. Alternatively, over-current protection devices 110A-110N may include an active device 114A-114N. Since each active device 114A-114N is identical in configuration and operation, only active device 114A will be described. Active device 114A includes a resistor 130A connected in series with a semiconductor switch 132A, such as a MOSFET. The voltage drop across the resistor 132A, which corresponds to the amount of current flowing in power supply line V100A, is processed in an amplifier/comparator 134A, which has its output connected to the control terminal of the semiconductor switch 132A. When the current in power supply line V100A exceeds a reference value (corresponding to the maximum acceptable current flow), the semiconductor switch 132A is shut off by the output of the amplifier/comparator 134A, thus preventing the excessive forward current from continuing to flow.
Fault protection devices 116A-116N provide an ORing function which prevents a fault, such as a short, in the associated power supply 102A-102N, which would cause a reverse current in the common power bus V100, from affecting the operation of the common load 104, by dragging down the common power bus V100. ORing devices 116A-116N may include a passive device, such as a diode 118A-118N, which would block any reverse current in the common power bus V100. However, such a diode may have a forward voltage drop across it that may be unacceptable for the power supply system.
Alternatively, a single N-channel MOSFET ORing device 120A-120N may be connected in series between the over-current protection device 110A-110N and the common power bus V100 to provide the ORing function. By connecting the source terminal of the MOSFET to the power supply side and the drain terminal to the common power bus V100, current cannot travel in a reverse direction, from the common power bus V100 to the power supply, unless the gate terminal is driven to the ON state by the ORing controller. As is known in the art, the ORing controller monitors the direction of current flow through the MOSFET, creates a sufficient gate voltage via a charge pump circuit to keep the MOSFET in the ON state when current is flowing in the forward direction, from the power supply to the common load and toggles the MOSFET to the OFF state if current begins to flow in the reverse direction. Such an ORing controller can be built using many different discrete components or an “off-the-shelf” controller, such as the MAX8535 controller from Maxim Integrated Products of Sunnyvale, Calif., may be used, with minimal external components, to provide the ORing function to the MOSFET.
Another alternative design of the ORing controller is shown at 122A-122N. This design includes a pair of MOSFETs connected in series, but in opposing directions to each other, between the over-current protection device 110A-110N and the common power bus V100. This design, in addition to providing the ORing function, provides a “soft start” feature, which limits the current demanded from the load at power-up. The soft start feature allows the power-up current to increase gradually by transitioning the MOSFETs at a predetermined rate from the OFF state to the ON state. Since the internal body diode in the MOSFET provides a forward current path through the MOSFET, even when the MOSFET is in the OFF state, the single MOSFET device 120A-120N cannot be used to provide the soft-start feature, which is achieved by connecting the second MOSFET in series with the first MOSFET, but in the opposite direction as the first MOSFET. An ORing controller, such as that described above, can be configured to ramp up the gate voltages for both MOSFETs gradually, thus allowing the pair to operate in the linear region during power-up.
While the above configurations of the over-current protection devices and ORing devices provide the desired protection features, the number of components needed to provide the over-current and ORing functions increases the cost of the system and degrades the electrical and thermal efficiency of the system.